1. Field of the Invention
The present invention relates to an AT-cut quartz resonator which is adapted so that a deviation of its resonance frequency from a target value, which is attributable to an error of the angle of cutting out a quartz substrate from an artificially grown quartz crystal, can be corrected through a simple modification of the electrode structure and hence kept within acceptable limits, thereby eliminating the addition of a temperature compensating circuit to the incorporation of the quartz resonator into an oscillation circuit in consumer-oriented electronic products and negating the need for adjustment of the temperature compensating circuit in industrial equipment.
2. Description of the Prior Art
Heretofore, quartz oscillators have been widely used as reference frequency generators for communications equipment, industrial equipment, consumer-electronics products, and so forth because of their excellent characteristics such as small size, high frequency accuracy and high frequency stability. Quartz resonators are also used in a wide variety of communications equipment in large quantities; for example, a plurality of quartz resonators are used to form a crystal filter, or a quartz resonator, an amplifier circuit and a temperature compensating circuit are used, in combination, to form a temperature compensated crystal oscillator (TCXO). The oscillation frequency of the crystal oscillator depends on the resonance frequency of the quartz resonator forming the oscillator. The quartz is a physically stable piezoelectric crystal, whose resonance frequency also has extremely high stability. In particular, an AT-cut quartz resonator has excellent temperature-frequency characteristics, and for this reason, it is frequently used in various fields.
The AT-cut quartz resonator has, as is well-known in the art, a pair of opposed electrode patterns for excitation deposited on both sides of a quartz substrate. The quartz substrate of an AT-cut quartz resonator is a Y plate having an angle .theta. about the X axis of 35 degrees 15 minutes obtained by cutting out it of artificially grown quartz crystal. The resonance frequency of the AT-cut quartz resonator depends on the thickness of the quartz substrate.
This quartz resonator is placed and sealed in a package provided with a required support structure to form one piezoelectric device. Further, a quartz oscillator is formed by inserting the quartz resonator in an oscillation circuit loop constructed using chip parts or the like on a printed board. While the AT-cut quartz resonator is small in the amount of the change in the resonance frequency with a change in temperature as compared with quartz resonators of other cutting angles as referred to above, it is known in the art that the AT-cut quartz resonator shows a cubic-like temperature-frequency characteristic with a point of inflection at about 27.degree. C., for example, as depicted in FIG. 5.
But the temperature-frequency characteristic varies with the angle .theta. for cutting out the quartz substrate from quartz crystal. That is, the temperature-frequency characteristic of the AT-cut quartz resonator becomes a function of the cutting angle .theta., forming the cubic curve having an inflection point at 27.degree. C.
FIG. 6 is a graph showing temperature-frequency characteristics of ordinary quartz resonators using three kinds of quartz substrates of slightly different cutting angles. For example, in Japan specifications about a reference frequency source of a pager (beeper) as consumer-oriented communication equipment require that a frequency deviation in a temperature range of -10.degree. C. to 50.degree. C. be held .+-.2 ppm. Assume, for instance, that the curve A in FIG. 6 is representative of the frequency deviation which meets the above mentioned specs. In this instance, the specs ought to be met by a quartz resonator using a quartz substrate cut out at an angle that provides the temperature characteristic corresponding to the curve A.
However, even if an attempt is made to cut out from quartz crystal a quartz substrate which meets such specs, a wide range of variations in the cutting angle of the AT-cut quartz substrate results from limitations on the machining accuracy of a cutter for cutting the quartz substrate out. With a greatly varied cutting angle, a frequency deviation of around .+-.5 ppm occurs in the temperature range of -10.degree. C. to 50.degree. C. On this account, percentages of good products that meet the specs of consumer-oriented equipment are as low as 20% to 30%, constituting a major factor in raising the cost of production of quartz resonators.
To meet the specs, it is general practice in the prior art that quartz resonators of frequency deviations exceeding .+-.2 ppm in the temperature range of -10.degree. C. to 50.degree. C. are each incorporated into a temperature compensated oscillation circuit added with a temperature compensating circuit. However, the temperature characteristic of every AT-cut quartz resonator shows a different temperature characteristic curve; therefore, the use of such a temperature compensating circuit for each quartz resonator involves an additional step of adjusting values of individual elements of the compensating circuit in the manufacture of consumer-oriented equipment, inevitably resulting in an increase in the cost of production.
Specifications about the quartz resonator for use in industrial communication equipment such as a cellular telephone also require that the frequency deviation in a range of -30.degree. C. to 75.degree. C. be held within .+-.2 ppm. It is impossible, however, to realize an AT-cut quartz resonator that meets such specs, now matter what cutting angle is chosen and no matter how much the cutting accuracy is improved.
For the reasons given above, a temperature compensating circuit is essential to an oscillator for use in industrial equipment. As in the case with consumer-oriented equipment, variations in the cut angle of the quartz substrate leads to dispersion in the temperature characteristics of quartz resonators, and values of individual elements of each compensating circuit need to be adjusted. Since this adjustment operation is extremely cumbersome, the addition of such a manufacturing process raises the cost of production of industrial equipment.
Such disadvantages as mentioned above ought to be completely overcome simply by raising a yield rate of quartz resonators. That is, if quartz resonators which fulfill various specs can be offered with high productivity and with a high yield rate, it will be possible to fabricate an oscillator for consumer-electronics products which follows the specs without inserting the temperature compensating circuit in the oscillation circuit; in industrial equipment, it will also be possible to eliminate the need for adjusting the temperature compensating circuit in the oscillation circuit. This ought to improve the productivity of various communication and electronic gears and cut their manufacturing costs.
With a view to obtaining an oscillator for consumer-oriented products with no temperature compensating circuit and obviating the need for making adjustments to the temperature compensating circuit in industrial equipment, the object of present invention is to provide AT-cut quartz resonators which do not exhibit a wide range of temperature characteristic variation, that is, AT-cut quartz resonators whose temperature characteristics fall within a range of .+-.2 ppm, for example, in the temperature range of -10.degree. C. to 50.degree. C.